Driver circuit and a driving method for a variable-reluctance motor

ABSTRACT

A variable-reluctance motor is driven by means of a driver circuit which is composed of a common switching device and switching devices corresponding to individual phases. The sum (total current i t ) of currents flowing through coils for the individual phases is detected. The duty ratio of a PWM signal for turning on and off the common switching device is computed in accordance with the deviation between the total current i t  and a current command i cmd  and the sign (positive or negative) of the deviation. Also, the on-off operation of the switching device corresponding to each phase is controlled depending on the excitation phase based on the rotor electrical angle and on whether the sign of the deviation is positive or negative. In this manner, current loop control is executed so that the detected total current i t  follows up the current command i cmd . Thus, the current is continuously controlled even during a period for the change of the excitation phase, so that the occurrence of a torque ripple can be restrained.

TECHNICAL FIELD

The present invention relates to a driver circuit and a driving methodfor a variable-reluctance motor (VR motor). More specifically, theinvention relates to a driver circuit and a driving method, in whichdrive control is effected by means of a circuit for motor currentcontrol including switching devices one more in number than the phasesof the motor.

BACKGROUND ART

A VR motor is a motor in which excitation current is supplied toexciting coils of a stator so that salient-pole teeth of the stator areexcited, a salient-pole tooth of a rotor is attracted by means of amagnetic force of attraction generated in the salient-pole teeth of thestator, and the rotor is rotated by means of the resulting rotatoryforce. This motor is provided with switching devices for supplyingexcitation current to the exciting coils for individual phases, and theswitching devices are opened and closed in response to the rotationalangle of the motor, whereby the exciting coils for the individual phasesare excited to rotate the rotor.

In the case of a three-phase VR motor with A-, B- and C-phases, forexample, an A-phase switching device is closed to connect an A-phaseexciting coil and a DC power source, thereby starting to supply current.When an A-phase salient-pole tooth attracts the salient-pole tooth ofthe rotor so that the rotor rotates through a predetermined angle, theA-phase switching device is opened to suspend current supplying. Then, aB-phase switching device is closed to excite a B-phase exciting coil.The motor is rotated in one direction by successively exciting the A-,B-, and C-phase coils in a like manner, thereafter. In reversing themotor, the motor can be reversed by exciting the A-, C-, and B- phasecoils in the order named.

In controlling the current flowing through each exciting coil of this VRmotor according to the pulse-width modulation system (PWM system),driver circuits must be formed independently for the individual phases.Therefore, each phase requires four switching devices or a combinationof two switching devices and two diodes. Thus, a driver circuit of theconventional VR motor requires use of a number of switching devices anddiodes, so that the driver circuit itself is increased greatly in cost,and requires two cables for each phase. Accordingly, the driver circuitbecomes more expensive, and its wiring entails more man-hours.

An arrangement improving this point has been disclosed in a patentapplication in Japan (Jpn. Pat. Appln. No. 4-84966). According to thispatent application, there is provided a driver circuit which requiresuse of only (N+1) switching devices where N is the number of phases ofthe VR motor.

The circuit diagram of FIG. 2 shows an example of the driver circuit ofthe three-phase VR motor in which the number of switching devices-isequal to "phase number+1."

In FIG. 2, reference numeral 1 denotes a rectifier circuit whichrectifies three-phase alternating currents R, S and T to generate a DCvoltage (main voltage) V. C1 designates a smoothing capacitor. In thisdriver circuit, a common series circuit is formed such that one end of acommon switching device Q1 for pulse width modulation (PWM) operation isconnected to a positive terminal of the rectifier circuit 1, and thecathode of a diode D1 is connected to the other end of the device Q1,the anode of the diode D1 being connected to a negative terminal of therectifier circuit 1.

Moreover, this driver circuit is provided with series circuits for theindividual phases, that is, the A-, B-, and C-phases, such that one endof each of switching devices (transistors) Q2, Q3 and Q4 foralternatively exciting the A-, B-, and C-phase coils is connected to thenegative terminal of the rectifier circuit 1, the other ends areconnected individually to the respective anodes of diodes D2, D3 and D4.The respective cathodes of the diodes D2, D3 and D4 are connected to thepositive terminal of the rectifier circuit 1.

The junctions of the switching devices Q2, Q3 and Q4 and the diodes D2,D3 and D4 of the series circuit for the individual phases are connectedto one ends of their corresponding exciting coils of the reluctancemotor, while the respective other ends of the exciting coils areconnected to the junction of the switching device Q1 and the diode D1 ofthe common series circuit.

As described above, the switching devices in this driver circuit includea common one (common switching device Q1) and one for each phase.

In FIG. 2, symbols ZA, ZB and ZC designate the impedances of the A-, B-,and C-phase coils of the VR motor, respectively. Also provided arecurrent detectors for detecting currents i_(a), i_(b) and i_(c) flowingthrough the individual coils. In the example shown in FIG. 2, currentdetecting resistors Ra, Rb and Rc are shown as detectors based oncurrent detecting resistances. Symbol i_(t) designates a total currentgiven by i_(t) =i_(a) +i_(b) +i_(c).

In connection with this arrangement, the drive of the VR motor, that is,excitation of the coils, taking the case of excitation of the A-phasecoil, will be described.

(1) When a positive voltage is applied to the A-phase coil to increasethe A-phase current i_(a) :

The A-phase switching device Q2 is turned on, the switching devices Q3and Q4 for the other phases are turned off, and the switching device Q1is turned on and off in response to a PWM signal. Thereupon, when theswitching device Q1 is on, the current i_(a) flows through the commonswitching device Q1, A-phase coil (Ra; ZA), and A-phase switching deviceQ2 in the order named, and the voltage V is applied to the A-phase coil,so that the current i_(a) flowing through the A-phase coil increases.When the switching device Q1 is turned off, on the other hand, energyaccumulated in the A-phase coil causes the current i_(a) to flow throughthe diode D1 of the common series circuit, A-phase coil (Ra; ZA), andA-phase switching device Q2 in the order named, and a voltage "0" isapplied to the A-phase coil.

Thus, if the duty ratio of the PWM signal for turning on and off thecommon switching device Q1 is ηa, an average voltage applied to theA-phase coil, in the process of applying the positive voltage to theA-phase coil, is equal to the product of the duty ratio ηa and the mainvoltage V, that is, (ηa×V).

(2) When a negative voltage is applied to the A-phase coil to reduce theA-phase current i_(a) :

In order to apply the negative voltage to the A-phase coil, all of theA-, B-, and C-phase switching devices Q2, Q3 and Q4 are turned off.

When the common switching device Q1, whose operating state is changed inresponse to the PWM signal, is on, the current i_(a) flows through thecommon switching device Q1, A-phase coil (Ra; ZA), and A-phase diode D2in the order named, and the voltage "0" is applied to the A-phase coil.

When the common switching device Q1 is off, on the other hand, thecurrent i_(a) flows through the diode D1 of the common series circuit,A-phase coil (Ra; ZA), and A-phase diode D2 in the order named, and avoltage "-V" is applied to the A-phase coil.

Thus, in the process of applying the negative voltage to the A-phasecoil, the average voltage applied to the A-phase coil takes a valueobtained by multiplying the difference between 1 and the duty ratio ηaby the product of the main voltage V and minus 1, that is, (1-ηa)×(-V).

Through the operations (1) and (2) described above, the A-phase excitingcurrent i_(a) is controlled by means of the PWM signal so as to followup a command current duping an A-phase excitation section. When themotor rotates so that the excitation phase changes to the B-phase, theswitching devices Q2 and Q3 are turned off and on, respectively, whichindicates only that the switching device Q3 serves in place of theswitching device Q2 in the case of A-phase excitation described above.Thus, the operation of the switching devices Q1 and Q3 and the voltageapplied to the B-phase coil have the same relationship as in the case ofthe A-phase. Likewise, when the motor rotates for C-phase excitation,the role of the switching device Q2 for the A-phase excitation is onlyreplaced with that of the switching device Q4, and thus the operationand the voltage applied to the C-phase coil is substantially the same asin the case of the A- and B- phase coils.

FIGS. 4A and 4B are diagrams for illustrating the relationships betweenthe coil currents i_(a), i_(b) and i_(c) for the individual phases,total current i_(t), and command current i_(cmd) in this driver circuit.

In the case of the A-phase excitation, the A-phase current i_(a) iscontrolled in accordance with the duty ratio ηa of the PWM signal, whichis settled depending on a current deviation equivalent to the differencebetween the command current i_(cmd) and the A-phase current i_(a), so asto rise and follow up the current command i_(cmd). When the excitationmode is changed from the A-phase excitation to the B-phase excitation inthe next stage, the B-phase current i_(b) rises and is controlled inaccordance with a duty ratio ηb, which is settled depending on a currentdeviation equivalent to the difference between the command currenti_(cmd) and the detected B-phase current i_(b). However, the fall of theA-phase current immediately after the change to the B-phase excitationis not controlled at all.

Since the switching device Q2 is off immediately after the start of theB-phase excitation, the current (last-transition current) flowingthrough the A-phase coil flows through the common switching device Q1,A-phase coil, and A-phase diode D2 in the order named, and the voltage"0" is applied to the A-phase coil when the common switching device ison. When the common switching device is off, on the other hand, thecurrent flows through the diode D1 of the common series circuit, A-phasecoil, and A-phase diode D2 in the order named, and the "-V" is appliedto the A-phase coil. As in the case of the aforesaid operation (2),therefore, the average voltage applied to the A-phase coil is given by(1-ηb)×(-V).

The A-phase current i_(a) is drastically reduced when the excitationphase is changed, and becomes 0 after the passage of a certain time(t_(ab)). In the section t_(ab) for the excitation phase change from theA-phase to the B-phase, the total current i_(t) flowing through themotor is equal to the sum of the A-phase last-transition current and theB-phase first-transition current.

As mentioned before, however, the falling A-phase current i_(a) is notcontrolled (because the average voltage for the A-phase is based not onthe duty ratio ηa to be settled depending on the A-phase currentdeviation, but on the duty ratio ηb to be settled depending on theB-phase current deviation), although the B-phase current i_(b) iscontrolled. Accordingly, the total current i_(t) of the motor is notcontrolled for the section t_(ab). After the A-phase current i_(a)becomes "0" when the section t_(ab) terminates, the B-phase currenti_(b) is equivalent to the total current i_(t) (=i_(b)).

The same also applies to the cases of excitation phase changes from theB-phase to the C-phase and from the C-phase to the A-phase. As shown inFIGS. 4A and 4B, the total current i_(t) of the motor is not controlledin specific sections t_(ab), t_(bc) and t_(ca) for phase changes.

In other sections than the excitation phase changing sections t_(ab),t_(bc) and t_(ca), as described above, the total current i_(t) is acurrent for each excitation phase, and this current is controlled so asto follow up the command current i_(cmd). In the phase changing sectionst_(ab), t_(bc) and t_(ca), however, only the first-transition current iscontrolled so as to follow up the command current i_(cmd), and thelast-transition current is not controlled. After all, the total currenti_(t) of the motor is not controlled, resulting in a torque ripple. InFIG. 4B, a fine line is used to indicate that the total current i_(t) ofthe motor is not controlled in sections t_(ab), t_(bc) and t_(ca) forchanges, and therefore, cannot be securely made coincident with thecommand current i_(cmd).

In the case where the driver circuit of the VR motor is a driver circuitin which the number of switching devices used for the control is equalto (phase number+1), as described above, the current deviationequivalent to the difference between the command current i_(cmd) and thedetected current for the phase concerned is obtained for each phase, andthe current for the phase concerned is controlled in accordance with theduty ratio for the phase concerned which depends on the currentdeviation. Accordingly, a current detector for detecting the current foreach phase must be provided for each phase. Moreover, there are sectionsfor the excitation phase changes in which the total current of the motorcannot be controlled, so that a torque ripple may be caused.

DISCLOSURE OF THE INVENTION

The object of the present invention is to provide a driver circuit and adriving method for a VR motor, which requires use of only one currentdetector, and can prevent the occurrence of a torque ripple.

In order to achieve the above object, according to a method of thepresent invention, a current detector, in a VR motor driven by a drivercircuit comprising switching devices one more in number than the phasesof the VR motor, is mounted in a position such that a total currentequal to the sum of currents flowing through individual coils of the VRmotor can be detected, and a current loop process is executed such thatthe total current detected by means of the current detector follows up acurrent command.

Further, a circuit according to the present invention comprises: arectifier circuit; a common series circuit including a common switchingdevice for PWM operation and a diode, one end of the common switchingdevice being connected to a positive terminal of the rectifier circuit,and the cathode and anode of the diode being connected to the other endof the common switching device and a negative terminal of the rectifiercircuit, respectively; a series circuit including switching devices foralternatively exciting the coils for the individual phases and diodes,one end of each of the switching devices being connected to the negativeterminal of the rectifier circuit, the other end being connected to theanode of corresponding diodes, and the cathode of each of the diodesbeing connected to the positive terminal of the rectifier circuit, thejunction of the switching device of the series circuit for each phaseand the corresponding diode being connected to one end of acorresponding exciting coil of the reluctance motor, a current detectorbeing mounted in a position such that a total current equal to the sumof currents flowing through the individual coils of the motor can bedetected, and connected between the junction of the switching device andthe diode of the common series circuit and the respective other ends ofthe individual exciting coils; and a control circuit for receiving acurrent value transmitted from the current detector, a command currentvalue, a main voltage value delivered from the rectifier circuit, and adetected value of the rotor electrical angle of the reluctance motor,computing the duty ratio of a pulse width modulation signal forswitching the switching device of the common series circuit, andoutputting a signal for controlling the switching operation of theswitching device in the series circuit for each of the phases.

According to the present invention, as described above, the totalcurrent equal to the sum of the currents flowing through the individualcoils of the motor is detected by the single current detector, and thesame current loop control as the conventional one is executed in amanner such that the detected total current follows up the currentcommand. Thereupon, the total current follows up the current command.Thus, the total current flowing through the motor is controlled evenduring a period for the change of the excitation phase, so that theoccurrence of a torque ripple can be restrained.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a driver circuit for a VR motor for carrying out a systemaccording to the present invention;

FIG. 2 is a diagram showing a conventional driver circuit which controlsthe current of the VR motor with use of switching devices one more innumber than the phases of the motor;

FIGS. 3A and 3B are diagrams for illustrating currents for individualphases and total current, respectively, according to one embodiment ofthe present invention;

FIGS. 4A and 4B are diagram for illustrating currents for the individualphases and total current, respectively, in the driver circuit shown inFIG. 2; and

FIG. 5 is a flow chart showing a current loop process executed by aprocessor of a control device for carrying out the method of the presentinvention.

BEST MODE FOR CARRYING OUT THE INVENTION

FIG. 1 is a circuit diagram showing a motor driver circuit according toone embodiment of the present invention. In FIGS. 1 and 2, like symbolsrefer to the same components. A common switching device Q1, switchingdevices (transistors) Q2, Q3 and Q4 for alternatively exciting A-, B-,and C-phase coils, diodes D2, D3 and D4, etc., are connected to arectifier circuit 1 in the same manner as in FIG. 2. The circuit of FIG.1 differs from the conventional VR motor driver circuit shown in FIG. 2in that only one current detector (shown as a current detecting resistorR) is used, and a total current i_(t) equal to the sum of currentsflowing through the individual phases is detected by means of thecurrent detector R. The hardware of control means for controlling thedrive of this VR motor, which is constructed in the same manner as thatof the conventional one, includes a processor, ROM, RAM, and input andoutput circuits, etc. Only the process executed by the processor of thecontrol means is different from the conventional one.

FIGS. 3A and 3B show the currents for individual phases and the totalcurrent, respectively, for the circuit shown in FIG. 1.

FIG. 5 is a flow chart showing a current loop process executed by theprocessor of the control means according to the one embodiment of thepresent invention (the circuit as shown in FIG. 1).

The processor of the control means, which executes the process shown inFIG. 5 for each predetermined period, first reads a current commandi_(cmd), and also reads the total current i_(t) obtained by the currentdetector R and a rotor electrical angle θ detected by a detector forreading the rotor position of the motor (Step S1). Then, the currentcommand i_(cmd) and the read total current i_(t) are compared. If thetotal current i_(t) is not higher than the current command i_(cmd), aduty ratio η of a PWM signal is obtained by subtracting the totalcurrent i_(t) from the current command icm_(d), multiplying theremainder by a proportional gain K of a current loop, and furtherdividing the product by a value V equivalent to the main voltage as theoutput of the rectifier circuit. In other words, the duty ratio η isobtained by making a computation according to equation (1) as follows(Step S3):

    η=(K/V)(i.sub.cmd -i.sub.t).                           (1)

If the total current i_(t) is higher than the current command i_(cmd),on the other hand, the duty ratio η of the PWM signal is obtained bymaking a computation according to equation (2) as follows (Step S4):

    η=1+(K/V)(i.sub.cmd -i.sub.t.                          (2)

Then, the excitation phase is settled in accordance with the rotorelectrical angle θ read in Step S1 (Steps S5 and S6). If the detectedtotal current i_(t) is less than or equal to the current command i_(cmd)in the case of A-phase excitation (Step S7), the aforesaid operation (1)must be performed to increase the total current i_(t). Accordingly, theswitching device Q2 corresponding to the A-phase is turned on, the PWMsignal is outputted so that the switching device Q1 is turned on and offwith the duty ratio η obtained in Step S3, and the other switchingdevices Q3 and Q4 are turned off, whereby the A-phase current isincreased to augment the total current i_(t) (Step S8). As mentionedbefore, the voltage applied to the A-phase coil is equivalent to themain voltage V as the output of the rectifier circuit 1 when theswitching device Q1 is on, and is "0" when the switching device Q1 isoff. At this time, therefore, an average voltage applied to the A-phasecoil is given as follows: ##EQU1##

If the detected total current i_(t) is higher than the current commandi_(cmd) (Step S7), on the other hand, the aforesaid operation (2) mustbe performed to reduce the total current i_(t). Accordingly, theswitching devices Q2, Q3 and Q4 are turned off, and the PWM signal isoutputted so that the switching device Q1 is turned on and off with theduty ratio η obtained in Step S4, whereby the A-phase current is loweredto reduce the total current i_(t) (Step S9).

As mentioned above, in this case, the voltage applied to the A-phasecoil is "0" when the switching device Q1 is on, and is "-V" when theswitching device Q1 is off. As a result, the average voltage takes avalue given by equation (4) as follows: ##EQU2##

As seen from equations (3) and (4) described above, the average voltageapplied to the A-phase coil takes a value equal to the product of theproportional gain K and a current deviation obtained by subtracting thetotal current i_(t) from the current command i_(cmd), and proportionalcontrol is executed in a manner such that the total current i_(t)follows up the command current i_(cmd), without regard to therelationship between the total current i_(t) and the current commandicm_(d).

If it is concluded in Step S6 that the excitation phase is the B- orC-phase, the same process (Step S10 or S13) as Step S7 for the A-phaseis executed, whereupon the current command i_(cmd) and the total currenti_(t) are compared. If the current command i_(cmd) is higher, or equalto i_(t) the switching device Q3 is turned on, while the switchingdevices Q2 and Q4 are turned off, in the case of B-phase excitation. Inthe case of C-phase excitation, the switching device Q4 is turned on,while the switching devices Q2 and Q3 are turned off. In this state, theswitching device Q1 is switched in response to the PWM signal with theduty ratio η obtained in Step S3 (Step S11 or S14). If the total currenti_(t) is higher than the current command i_(cmd), on the other hand, theswitching devices Q2, Q3 and Q4 are turned off, and the switching deviceQ1 is switched with the duty ratio η obtained in Step S4 (Steps S12 andS15).

The above-described processes are executed for each predetermined periodso that the voltage of the value obtained by multiplying the currentdeviation, or the difference between the current command i_(cmd) and thedetected total current i_(t), by the proportional gain K of the currentloop is applied to the excitation phase, whereby the total current i_(t)is controlled so as to follow up the command current icm_(d).

A control circuit (not shown in FIG. 1) is used to obtain the duty ratioη of the PWM signal in Steps S3 and S4 and obtain control signals forturning on and off the switching devices Q2, Q3 and Q4 for theindividual phases in Steps S8, S9, S11, S12, S14 and S15. In order tocompute these values, this control circuit receives the current commandvalue i_(cmd), detected total current i_(t), rotor electrical angle θ,and output (main voltage V) of the rectifier circuit 1.

In FIG. 3B, a solid line is used to indicate that the total currenti_(t) of the motor is controlled in sections t_(ab), t_(bc) and t_(ca)for changes, and therefore, can be securely made coincident with thecommand current i_(cmd).

According to the present invention, it is necessary only to detect thetotal current equal to the sum of the currents flowing through the coilsof the individual phases, so that only one current detector willsuffice. Since the current loop process is executed so that the totalcurrent follows up the current command, moreover, the current flowingthrough the motor is continuously controlled even during a period forthe change of the excitation phase, so that the occurrence of a torqueripple can be restrained.

We claim:
 1. A driver circuit for a variable-reluctance motor havingindividual exciting coils for respective phases, said driver circuitcomprising:a rectifier circuit having positive and negative terminals; acommon series circuit for delivering a main voltage from said rectifiercircuit to the individual coils of the motor, said common series circuitincluding a common switching device for pulse width modulation operationand a first diode, one end of said common switching device beingconnected to the positive terminal of said rectifier circuit, and acathode and an anode of said first diode being connected to the otherend of said common switching device and the negative terminal of saidrectifier circuit, respectively; a series circuit including switchingdevices for alternately exciting the coils for the individual phases,and second diodes, respective first ends of said switching devices ofsaid series circuit being connected to the negative terminal of saidrectifier circuit, the respective other ends being connected torespective anodes of said second diodes, and each of respective cathodesof said second diodes being connected to the positive terminal of saidrectifier circuit; the junction of the switching device of the seriescircuit for each phase and the corresponding second diode beingconnected to one end of the corresponding exciting coil of the motor; acurrent detector to detect a current value of a total current equal tothe sum of currents flowing through the individual coils of the motorand connected at one end to the junction of the common switching deviceand said first diode of said common series circuit and at the other endto the respective other ends of said exciting coils; and a controlcircuit to receive the current value transmitted from said currentdetector, a command current value, a value of the main voltage deliveredfrom said rectifier circuit, and a detected value of the rotorelectrical angle of the reluctance motor, said control circuit computinga duty ratio of a pulse width modulation signal for switching saidcommon switching device of said common series circuit, and outputting asignal for controlling the switching operation of the correspondingswitching device in the series circuit for each said phase.
 2. Thedriver circuit for a variable-reluctance motor according to claim 1,wherein said current detector is located between a common junction ofthe respective other ends of said individual exciting coils and thejunction of said common switching device and said first diode of saidcommon series circuit.
 3. The driver circuit for a variable-reluctancemotor, according to claim 1, wherein said current detector is aresistor.